Unsteady Aerodynamics Research Articles

High-Speed Virtual Flight Testing Platform for Performance Evaluation of Pitch Maneuvers

To research serious nonlinear coupling problems among aerodynamics, flight mechanics, and flight control during high maneuvers, a virtual flight testing platform has been developed for a large-scale, high-speed wind tunnel, based on the real physical environment, and it can significantly mitigate risks and reduce the costs of subsequent flight tests. The platform of virtual flight testing is composed of three-degrees-of-freedom model support, measuring devices for aerodynamic and motion parameters, a virtual flight control system, and a test model. It provides the ability to realistically simulate real maneuvers, investigate the coupling characteristics of unsteady aerodynamics and nonlinear flight dynamics, evaluate flight performance, and verify the flight control law. The typical test results of a pitch maneuver with open-loop and closed-loop control are presented, including a one-degree-of-freedom pitch motion and a two-degrees-of-freedom pitch and roll motion. The serious pitch and roll-coupled motion during a pitch maneuver at a high angle of attack is revealed, and the flight control law for decoupled control is successfully verified. The comparison of the test results and the flight data of a real pitch maneuver proves the reliability and capability of virtual flight testing.

Mass and Force Lumping: An Essential Enhancement to the Intrinsic Beam Finite Element Discretization

This paper introduces the novel application of the mass and force lumping technique to enhance the finite element discretization of the fully intrinsic beam formulation. In our aeroelastic system model, 2-D unsteady aerodynamics were incorporated alongside simple calculations for thrust and gravity. Through the central difference discretization method, the discretized system was thoroughly examined, shedding light on the advantages of the mass and force lumping approach. With the use of a first-order lumping method, we successfully reconstructed the inertia matrices, external forces, and moments. The resulting equations are more systematically structured, facilitating the extraction of a regular state-space linear system using the direct index reduction method post-linearization. Numerical results further confirm that the proposed techniques can effectively capture the nonlinear dynamics of aeroelastic systems, enabling equation reconstruction and leading to significant benefits in system order reduction and flight dynamical analysis.

On the characteristics of the wake of a wind turbine undergoing large motions caused by a floating structure: an insight based on experiments and multi-fidelity simulations from the OC6 project Phase III

Abstract. This study reports the results of the second round of analyses of the Offshore Code Comparison, Collaboration, Continued, with Correlation and unCertainty (OC6) project Phase III. While the first round investigated rotor aerodynamic loading, here, focus is given to the wake behavior of a floating wind turbine under large motion. Wind tunnel experimental data from the UNsteady Aerodynamics for FLOating Wind (UNAFLOW) project are compared with the results of simulations provided by participants with methods and codes of different levels of fidelity. The effect of platform motion on both the near and the far wake is investigated. More specifically, the behavior of tip vortices in the near wake is evaluated through multiple metrics, such as streamwise position, core radius, convection velocity, and circulation. Additionally, the onset of velocity oscillations in the far wake is analyzed because this can have a negative effect on stability and loading of downstream rotors. Results in the near wake for unsteady cases confirm that simulations and experiments tend to diverge from the expected linearized quasi-steady behavior when the rotor reduced frequency increases over 0.5. Additionally, differences across the simulations become significant, suggesting that further efforts are required to tune the currently available methodologies in order to correctly evaluate the aerodynamic response of a floating wind turbine in unsteady conditions. Regarding the far wake, it is seen that, in some conditions, numerical methods overpredict the impact of platform motion on the velocity fluctuations. Moreover, results suggest that the effect of platform motion on the far wake, differently from original expectations about a faster wake recovery in a floating wind turbine, seems to be limited or even oriented to the generation of a wake less prone to dissipation.

Discovering optimal flapping wing kinematics using active deep learning

This paper focuses on the discovery of optimal flapping wing kinematics using a deep learning surrogate model for unsteady aerodynamics and multi-objective optimisation. First, a surrogate model of the unsteady forces experienced by a 3-D flapping wing is built, based on deep neural networks. The model is trained on a dataset of randomly generated kinematics simulated using direct numerical simulation (DNS). Once trained, the neural networks can quickly predict the unsteady lift and torques experienced by the wing, using sparse information on the kinematics. This fast surrogate model allows multi-objective optimisation to be performed. The resulting Pareto front consists of new kinematics that may be very different from the kinematics of the initial dataset. A few arbitrarily chosen kinematics on the Pareto front are thus simulated using DNS and used to enhance the database. The new dataset is used to train again the networks, and this active deep learning/optimisation framework is performed until convergence, obtained after only two iterations. Overall, this method reduced the cost of optimisation by 83 %. Results reveal two distinct families of motions. Kinematics promoting high efficiency are characterised by large stroke amplitudes and relatively low angles of attack, as observed for fruit flies, honeybees or hawkmoths. For those, lift production is driven by quasi-steady effects and the formation of a stable leading edge vortex. Kinematics promoting high lift are characterised by small stroke amplitudes and high angles of attack, reminiscent of mosquitoes. Lift production is driven by the rapid generation of vorticity at the trailing edge.

Prediction and suppression of chaos following flutter in wind turbines

Chaotic motion in a fluttering wind turbine blade is investigated by the development of an efficient analytical predictive model that is then used to suppress the phenomenon. Flutter is a dynamic instability of an elastic structure in a fluid, such as an airfoil section of a wind turbine blade. It is presently modelled using generalised two degree of freedom coupled modes of a blade airfoil section (pitch and plunge) combined with local unsteady aerodynamics, based on flutter derivatives and a continuous bilinear lift curve under damping. The mode coupling causes instability and limit cycle flutter due to a Hopf bifurcation. Following the critical flutter speed, the response can transition to chaos through successive other bifurcations like period doubling. New closed-form conservative analytical conditions for chaos following blade flutter are identified and discussed for the wind turbine section taking into account the blade geometry and optimal design of the wind turbine. These predictions are numerically verified for a range of conditions including stall slope and damping. The results confirm that chaos following blade flutter can occur due to nonlinearities in the aerodynamics, i.e. due to a bilinear lift law. This phenomenon is then suppressed to unrealistically high wind speeds and/or eliminated by quantified variation of system parameters using the predictive model. The results show that small changes in tip speed ratio (−15%), and stall slope factor (−17%) can eliminate or suppress chaos following flutter, while, in general, larger magnitude changes in dynamic parameters (i.e. mass, inertia > 81%, stiffness > 97%, damping > 100%) are required to achieve the same, by detuning the coupled plunge and pitch natural frequencies or damping out overlapping parametric resonances. These results also highlight that the analytical predictions can remarkably be generalized to any parameter set and provide almost instantaneous calculations representing many thousands of numerical simulations from many bifurcation diagrams (computational acceleration factor of 107 times). General insight is also provided into the occurrence and suppression of airfoil chaos following flutter in aeroelastic structures like wind turbines.

An Aerodynamic Correction Technique for the Unsteady Subsonic Wing–Body Interference Model

This paper investigates a novel correction technique for the unsteady subsonic wing–body interference model. The correction technique considers the unsteady forces on the lifting boxes and the body elements of an idealized aircraft model. The chosen simulation model was a passenger aircraft, and the transonic unsteady aerodynamics in sinusoidal pitch motion at four different frequencies are analyzed. The unsteady aerodynamics of the uncorrected DLM (doublet lattice method), ECFT (enhanced correction factor technique) and the new unsteady wing–body correction method are compared to the unsteady CFD simulation results. The results show that when the frequency is small, the new unsteady wing–body correction method can obtain certain benefits in terms of accuracy, for the lifting boxes and the body elements as well.

Assessment of yaw-control effects on wind turbine-wake interaction: A coupled unsteady vortex lattice method and curled wake model analysis

Downstream wind turbines in a wind farm are affected by the wake of the upstream turbine. Therefore, wake steering has been introduced to alleviate the wake effect by yawing the shaft of the upstream wind turbine. In this scenario, the upstream and downstream wind turbines encounter non-uniform inflow in wake steering. In this study, the unsteady vortex lattice method (UVLM) was coupled with the curled wake model to consider the unsteady aerodynamics resulting from the wake steering. The proposed method was validated with the National Renewable Energy Laboratory (NREL) Phase VI experiment and Large Eddy Simulation (LES) data simulated with two-turbine case. Three NREL 5 MW wind turbines were simulated in wake steering conditions, and the power performance of the individual turbines and their root flapwise bending moments were investigated. The flow field investigation indicated that the counter-rotating vortices significantly influence the effective angle of attack and aerodynamic loads. In addition, a secondary effect was observed on the effective angle of attack of the rotor and wake deflection. The simulation results demonstrated the applicability of the UVLM in wake steering and its potential for analyzing the wake effects experienced by multiple wind turbines in a wind farm.

Numerical verification of the dynamic aerodynamic similarity criterion for wind tunnel experiments of floating offshore wind turbines

Model tests and real-time hybrid simulation tests of floating offshore wind turbines have recently been extensively conducted to explore the aero-hydro-servo-elastic coupling mechanism. The traditional thrust similarity criterion under the Froude scale cannot satisfy the dynamic aerodynamic similarity in the wave basin experiments. Meanwhile, the traditional model wind turbine in model tests shows huge differences in Reynolds number and significant deviations in the optimal tip speed ratio, and the dynamic performance is different from that of the prototype wind turbine. Therefore, a new criterion based on the mapping of the optimal tip speed ratio is proposed, and a new model wind turbine is designed to physically reproduce the dynamic aerodynamics of the prototype wind turbine accurately in wind tunnel experiments. Then the dynamic aerodynamic similarity in this criterion is numerically verified and compared with that in the traditional criterion. The simulation is performed in steady and unsteady inflows by the open-source OpenFAST. The platform motions of the prototype and model wind turbine are inflow-motivated and program-forced, respectively. The calculation results show that the dynamic thrust, power and their coefficients in the new criterion all maintain better similarity compared to those in the traditional criterion. This study is important for wind tunnel model tests and RTHS tests to accurately obtain unsteady aerodynamics, and it can guide the aero-hydro-structure load evaluations and turbine-floater-mooring integrated designs for large-scale floating offshore wind turbines.

Numerical study of the unsteady blade root aerodynamics of a 2 MW wind turbine equipped with vortex generators

Abstract. In order to design vortex generators for modern multi-megawatt wind turbines accurately, the 3D behaviour of the boundary layer has to be considered. Due to the rotation of the blade, the lift-enhancing rotational augmentation has a considerable impact, especially in the inner blade sections. To investigate the interaction of vortex generators and rotational augmentation, high-fidelity computational flow simulations by means of unsteady Reynolds-averaged Navier–Stokes methods are presented for a rotating blade of a generic 2 MW horizontal-axis wind turbine. The inner blade section is analysed with and without vortex generators for two different pitch settings, including one causing largely separated flow. Two ways of placing the vortex generators on the blade with different radial starting positions are investigated in order to find out if the coexistence of the two lift-enhancement methods (i.e. rotational augmentation and vortex generators) is beneficial. All simulations are performed with the flow solver FLOWer, and the vortex generators are modelled by the introduction of source terms into the computational domain through a so-called BAY (Bender–Anderson–Yagle)-type model. For the case without vortex generators, it is found that the strength of rotational augmentation largely depends on the effective angles of attack (i.e. the pitch setting). For the case with lower effective angles of attack, rotational augmentation is a cyclic phenomenon, whereas for the case with higher effective angles of attack, it generates large loads in the inner root section due to a constant centrifugal pumping mechanism in time. The results from the cases with vortex generators display a rather destructive interaction of vortex generators and rotational augmentation on the torque. For low effective angles of attack and thus attached flow conditions, vortex generators exhibit slight losses compared to the case without vortex generators, as they inhibit spanwise flow through rotational augmentation. For high effective angles of attack, the vortex generators placed over 30 % of the blade produce an increase of 3.28 % in torque compared to the case without vortex generators and high centrifugal pumping.

Kinetic Equation Study on Momentum Conservation of Aerodynamics Incompressible Fluid of High-Altitude Aircraft

This paper introduces an integrated aerodynamic design method for aircraft based on numerical solution of hydrodynamics and electromagnetics equations. By using Pro/Engineer, ANSA, Fluent and MATLAB, the geometric shape, unstructured fluid mesh, hydrodynamic parameters and unpowered flight trajectory data of an aircraft were processed and calculated. In this paper, N-S equations are studied, and a test scheme for a hydrodynamic material aerodynamic model is established. Then, relevant parameters of the aircraft are tested and studied. The feasibility of this method for unsteady aerodynamics analysis of aircraft is verified.

Aerodynamic Characteristics of a Z-Shaped Folding Wing

Z-shaped folding wings have the potential to enhance the flight performance of an aircraft, contingent upon its mission requirements. However, the current scope of research on unmanned aerial vehicles (UAVs) with Z-shaped folding wings primarily focuses on the analysis of their folding structure and aeroelasticity-related vibrations. Computational fluid dynamics methods and dynamic meshing are employed to examine the folding process of Z-shaped folding wings. By comparing the steady aerodynamic characteristics of Z-shaped folding wings with those of conventional wings, this investigation explores the dynamic aerodynamic properties of Z-shaped folding wings at varying upward folding speeds. The numerical findings reveal that the folding of Z-shaped folding wings reduces the lift-to-drag ratio, yet simultaneously diminishes the nose-down pitching moment, thereby augmenting maneuverability. Concerning unsteady aerodynamics, the transient lift and drag coefficients of the folded wing initially increase and subsequently decrease as the folding angle increases at small angles of attack. Likewise, the nose-down pitching moment exhibits the same pattern in response to the folding angle. Additionally, the aerodynamic coefficients experience a slight decrease during the initial half of the folding process with increasing folding speed. Once the wing reaches approximately 40°~45° of folding, there is an abrupt change in the transient aerodynamic coefficients. Notably, this abrupt change is delayed with higher folding speeds, eventually converging to similar values across different folding speeds.

Unsteady Aerodynamics of the Retropropulsion Reentry Burn of Vertically Landing Launchers

During the vertical descent and landing of a launcher first stage with the aid of retropropulsion, commonly two main propulsive deceleration maneuvers are performed: the reentry burn in high altitudes at hypersonic to supersonic speeds and the landing burn shortly before touchdown at transonic to subsonic speeds. In the frame of the EU-funded H2020 project Retro Propulsion Assisted Landing Technologies (RETALT), the unsteady aerodynamics of those retropropulsion phases were studied. This paper presents results of experiments performed in the Hypersonic Wind Tunnel Cologne on the hypersonic reentry burn. The exhaust plume was simulated with pressurized air. Proper orthogonal decomposition was performed on high-speed schlieren videos, and spectral analyses of the time histories of the resulting modes were compared to the frequency content found in high-frequency pressure measurements. Dominant frequencies were found in the proper orthogonal decomposition modes for one and for three active engines. In the pressure measurements, dominant frequencies could only be observed for three active engines. The normalized pressure fluctuations are in the range of 0.002–0.012. Additionally, a good scaling of the pressures on the base area and in the wake of the configuration with the total pressure downstream of the bow shock could be confirmed, in the sense that the ratio of the local surface pressure to the total pressure downstream of the bow shock match for varying freestream Mach numbers.

Unsteady Aerodynamic Characteristics of The Flapping Wing Micro Air Vehicle (MAV)

Flapping wing propulsion is center of attention in the field of micro air vehicle research in recent years. Knowledge in unsteady aerodynamics is insufficient to design an efficient wing that produces the required lift and thrust force for the flapping wing micro air vehicle. The unsteady aerodynamic effects on aerodynamic wing design parameters were discussed in this paper. The effect of aspect ratio and the wing planform shape on lift and thrust force were determined experimentally. The experiments were conducted in an open jet low speed wind tunnel having test section 350 mmx350 mm. A rack and gear type new flapping mechanism was designed and fabricated and this was used as test bed for all the experiments. Also an experiment was conducted to determine the effect of torsional flexibility of the wing. The result shows that the chord wise flexible wing produces more lift and thrust than the other wing.

Dynamic process and dynamic-stall phenomenon on blade sections of a floating horizontal-axis wind turbine caused by the platform's pitch motion

Floating horizontal-axis wind turbines (FHAWTs) experience six degrees of freedom motion in space with their floating platforms. Such dynamic process may induce complex dynamic flow and variable local load on the blades, however, still insufficiently understood from a perspective of unsteady aerodynamics. In this study, the local aerodynamic load in the dynamic process induced by the platforms’ pitch motion is investigated by computational fluid dynamics. Our results indicate that the axial force of each blade section fluctuates significantly over time, with varying levels of fluctuation among the blade sections. The pressure distributions around the blade sections also vary significantly. Additionally, we observe the light dynamic stall phenomenon that cause stronger lift in upstroke and weaker lift in downstroke, and the flow separation and reattachment process during the dynamic stall demonstrates a delayed response relative to instantaneous changes in the angle of attack. The load variations due to the dynamic process and dynamic stall can lead to severe fatigue and extreme loads on the surface and structure of the blades. These findings suggest that quasi-steady analysis, which neglects the dynamic process, is not sufficient for accurately predicting the blade loads under pitch motion. Furthermore, we find that the fatigue-load properties vary with location on a blade section, with the leading and trailing edges being more susceptible to fatigue problems. These results highlight the importance of considering the potential harmful effects of pitch motion in the design of FHAWTs.

Effect of wing planform on plunging wing aerodynamics

The unsteady aerodynamics of three plunging wings with different planform shapes was investigated in water tunnel experiments by means of force/moment and three-dimensional volumetric velocity measurements at a post-stall angle of attack. In contrast with a rectangular wing, a tapered wing does not exhibit a local minimum of the time-averaged lift coefficient at a reduced frequency of k≈ 1.7. This is due to the oblique shedding of trailing-edge vortices, which prevents the coupling with leading-edge vortices. The wing with a round-tip exhibits the same double maxima that correspond to the subharmonic and the fundamental frequency of vortex shedding in steady freestream. However, combined leading-edge and tip vortex produce larger time-averaged lift force with increasing reduced frequency. The variation of the bending moment is similar to that of the lift force in general. However, the mean bending moment arm shifts outboard with increasing reduced frequency due to the increased three-dimensionality of the vortex filaments and the tip-vortex effect. Maximum lift and bending moment become more similar for all three wings at higher reduced frequencies as the added mass effect becomes more significant.

Mimicking the Mechanism of Lift Generation in Dragonflies

Human pursuit of flight has learnt many lessons from nature and a few adaptations have led to phenomenal progress. Insect flight uses complex unsteady aerodynamics to generate lift in addition to producing thrust. A close look at the dragonfly provides some clues to mimic its flight using passive means by separating propulsion and lifting mechanisms. Though the dragonfly can independently flap each of its wings to generate lift and thrust, during forward flight, fore-wings alone flap while the aft-wings remain nearly steady. We adopt this as basis for new passive configurations to generate steady lift. Conceptually, we replace fore-wings with a cylinder that is known to shed vortices, to mimic flapping. A simple plate replaces the aft-wings. Flow analysis of such a configuration at Reynolds Number of 7500 revealed a steady flow topology featuring a trapped vortex over the plate, resulting in steady lift to drag ratio of 3. A further optimized configuration consisting of a semi-circular cylinder upstream and a cambered plate downstream resulted in lift to drag ratio of 14.5, which is considerable for these flow regimes, opening up new possibilities for design of nano-flight vehicles.

Experimental investigation on turbulence effects on unsteady aerodynamics performances of two horizontally placed small-size UAV rotors

Wind turbulent disturbance is one of the critical challenges to flying unmanned aerial vehicles (UAVs); especially when the size, weight, and power on board such aircraft are limited. Therefore, to shed light on such turbulent wind effects on UAVs' aerodynamic performances, wind tunnel experiments were conducted on the single- and multi-rotor configurations by varying the turbulence intensity up to 15%, while setting the wind velocity at 5 m/s. The turbulent disturbances were generated inside the wind tunnel using two different passive grids. One is with 29% blockage, and the other is 49%. As the single-rotor experimental tests were conducted, the onset turbulence was found to give rise to a significant change in thrust and the power coefficient in the presence of a lower revolution per minute (rpm). However, at higher rpm, only the wind effect is observed to be the dominant contributor to the aerodynamic performance. Additionally, three different rotor spacings are experimentally examined to measure the propeller wake interactions at the same velocity and different onset turbulence intensities. It was experimentally found that a significant wake interaction occurs between 0.5 D (diameter) and 1.25 D below the propellers, depending on the rotor spacing and the flow conditions. The present work opens up an experimental approach to examine the unsteady aerodynamic performance of multi-rotor UAVs in the presence of wind disturbances, i.e. at different turbulence intensities.

Experimental investigation of flow structure over pitching airfoil in the wake of circular cylinder

The unsteady aerodynamics play an important role in fluid-structure interaction due to its engineering applications. These types of research can be utilized to improve the fundamental analysis of bluff body-airfoil wake interaction and to achieve better design and more efficiency in engineering applications. This paper aims to conduct experimental tests over parameter space to investigate flow around a NACA0012 pitching airfoil in the wake of a circular cylinder. The one-dimensional hot-wire anemometer probe was used in an open-type wind tunnel to examine the effect of the cylinder-airfoil gap, the amplitude of pitching oscillation, and reduced frequency on the time-averaged flow characteristics in Reynolds number Rec=105. The results show that the increase of the reduced frequency at the α0=5 has a negligible effect on the flow characteristics, while its increase at the α0=10 is consequential. Pitching oscillation creates a region with a higher fluctuation in the wake center of the upstream cylinder, whose width grows with the increase in the amplitude of pitching oscillation. In addition, the distributions of the turbulent flow characteristics for GD=6and10 cases in xc=2.4 are approximately similar but different from the GD=2 case. The analytical results presented in this paper provide basic information about cylinder-pitching airfoil wake interaction.

The assessment of a fast computational method in predicting the unsteady loads of vertical axis wind turbines undergoing floating motion

The low center of gravity and low center of thrust of Vertical Axis Wind Turbines (VAWT) are interesting characteristics when considering Floating-Offshore Wind Turbine (FOWT) application. The motion due to the floating platform adds extra complexity to the unsteady aerodynamics of VAWT. Hence, both numerical and experimental studies become very challenging. This paper focuses on the assessment of TubinesFoam, a fast tool consisting of the actuator line method (ALM) embedded in OpenFoam. Then, instead the typical bladed-solved Navier–Stokes equations, the ALM inserts the blade forces into the field as body forces into the momentum equation. A modified version of the turbinesFoam library that includes surge motion for multiple rotors has been evaluated by comparison with an experimental set of data from a twin-rotor performing surge in a wind tunnel. This paper includes the main equations describing the kinematics implemented in the numerical code, as well as a convergence analysis of mesh size, time step and surge cycles. The numerically predicted thrust forces agreed with the experimental results for both investigated tip speed ratios for the case with no surge motion. The numerical and experiment results including surge motion indicated a minimal influence of the surge motion for the cases evaluated here. The last verification using CFD computations from the literature showed that the turbinesFoam tool properly captures the main features of the surge motion. This verified numerical method is a promising tool to understand the effects of geometrical parameters on the performance and the wake development of VAWTs farms in floating-offshore environments.

Interaction between Instabilities in Vertical-Axis Turbine Blades

This work investigates five different instability mechanisms that may occur and interact in straight-bladed H-rotor vertical-axis turbines. Specifically, it considers the static instabilities of divergence and centrifugal buckling as well as the dynamic instabilities of flutter, main resonance, and parametric resonance. This is done by applying Theodorsen’s model for unsteady aerodynamics with traditional finite element methods. The resulting model comprises bend-twist-coupled elements that inherently account for fluid and centrifugal effects. Comparisons with experimental results in the literature are used to assess the accuracy and limitations of the model. Eigenvalue analysis is used on time-averaged equations to predict divergence, flutter, and centrifugal buckling, as well as identify regions where main and parametric resonance may be excited. Time history analysis is used to provide further insight and show main and parametric resonance explicitly. Stability plots are constructed using both methods of analysis and presented in the forcing parameter space. Interaction between the different instability mechanisms is observed, including a region of parametric stabilization that is found to extend beyond the expected boundaries of both divergence and flutter.